The present invention relates to a method for controlling an extrusion line for formed insulation cables, and more particularly to such a cable extrusion line control system with controlled variables such as the outside diameter and electrostatic capacity of foamed insulation extruded and coated on the cable being multivariables.
Communication cables are generally composed of a conductor with a coating of insulation extruded thereon having a small electrostatic capacitance and hence a small dielectric constant to meet the demand for a reduced transmission loss caused by the cable. The insulation is also required to support the cable itself. For these reasons, polyethylene foams (PEF) with a multiplicity of cells contained in polyethylenes are widely used as insulation for communication cables.
FIGS. 1 and 2 of the accompanying drawings illustrate an extrusion line 10 for foamed insulation cables with insulation made of foamed polyolefin resins. The extrusion line 10 has a wire drawing unit 12 for elongating a wire rod 11 unwound from a wire stack 10a into a cable conductor 13 having a smaller diameter than that of the wire rod, and an annealer unit 14 for annealing the cable conductor 13 with current annealing.
The cable conductor 13 is then guided through a first dancer roller 15 preferably to an inductive preheater 16 and thereafter continuously to an extruder 17. The preheater 16 serves to preheat the cable conductor 13 for allowing insulation which be coated later to be uniformly foamed and also to be held in intimate contact with the cable conductor 13. The extruder 17 has a hopper 18 through which a foaming insulation compound composed of a polyolefin resin and an organic foaming agent is supplied to the extruder 17. The supplied compound is forced into a crosshead 21 by a screw 20 disposed in a cylinder 19, as shown in FIGS. 3 and 4. The screw 20 is driven by a motor 22 to rotate about its own axis. A cylinder heater 23 comprises a plurality of separate heater units 23-1 through 23-4 surrounding the cylinder 19. The crosshead 21 is heated by a crosshead heater 24 composed of a pair of crosshead heater units 24-1, 24-2. When the screw 20 rotates, and the cylinder heater 23 and the crosshead heater 24 are heated, the foaming insulation compound is heated at a temperature higher than the decomposition temperature of the foaming agent and is fed out from a restrictor 25 of the crosshead 21 in a direction normal to the axis of the restrictor 25. The foaming insulation compound is then extruded and coated onto the cable conductor 13 at a nipple 26.
Compressed gas contained in the foaming insulation compound under high pressure in the extruder 17 is expanded in the compound when the compound is extruded into the atmosphere. The expanded gas forms cells in the compound which have maximum volumes when the pressure therein is equalized to the atmospheric pressure. By cooling the foamed insulation, the pressure in the cells is reduced to thereby allow the foamed insulation to shrink in volume. To effect such cooling, the extrusion line includes a movable cooler unit 27 comprising a water trough 29 with a movable water trough 28 slidably disposed at an upstream end thereof. The distance between the movable water trough 28 and the crosshead, particularly its nipple 26, is adjusted to suppress the growth of chemical foaming in the insulation by water-cooling the latter in surrounding relation, and also to form a barrier on the outer peripheral surface of the resin for preventing foaming therein. The degree at which the foaming gas is expanded in the foamed insulation can thus be adjusted to gain a desired outside diameter and electrostatic capacity of the insulation.
The cooler unit 27 may be immovable with the amount and temperature of water contained therein being controllably variable.
The extrusion line 10 has an outside-diameter gage 30 for measuring the outside diameter of the insulation formed, and an electrostatic-capacity sensor 31 for measuring the electrostatic capacity of the insulation. The outside-diameter gage 30 may be a laser outside-diameter gage, type M501B, with the accuracy of 1 micron, manufactured by Anritsu Denki K.K., Japan. The electrostatic-capacity sensor 31 may be a sensor KI-700CGA (sensor KG 500) manufactured by BETA Company, Bucks in High Wycombe, England, the sensor being capable of measuring the electrostatic capacity (PF/m) of the formed insulation cable with the accuracy of .+-.0.1 PF/m.
In addition to the above gage 30 and the sensor 31, the extruder 17 has a plurality of cylinder-temperature sensors TM1 through TM4, and the crosshead 21 has a resin-temperature sensor TM5, a crosshead-temperature sensor TM6, and a resin-pressure gage PM (type CZ-1P manufactured by Rikagaku Kogyosha, Japan).
The foamed insulation cable is drawn by a withdrawal unit 32 and wound around a drum by a cable winder 34 through a second dancer roller 33.
In the manufacture of foamed insulation cables, it is necessary that the outside diameter (D) of the cable and the elestrostatic capacity (C) of the insulation be uniform in the longitudinal direction of the cable.
To provide a desired outside diameter and electrostatic capacity through controlling the extrusion process on the extrusion line 10, it has been customary practice to control the RPM of the screw 20 in the extruder 17 and the voltage applied to the preheater 16 in proportion to the speed at which the cable is withdrawn by the withdrawal unit 32, that is, the line speed.
The elestrostatic capacity (C) has been controlled by moving the movable water trough 28, while the outside diameter (D) has manually been ajusted. The manual adjustment of the outside diameter (D) has been carried out empirically by varying the RPM of the extruder screw 20 and the extruder temperature. In the actual practice, however, movement of the movable trough 28 results in a change in the electrostatic capacity and simultaneously in a change in the outside diameter. Although this control can control the electrostatic capacity at will, the outside diameter cannot be controlled since it undergoes a departure from the desired value. As a consequence, it has been quite difficult to control the electrostatic capacity and the outside diameter independently of each other with a view to manufacturing cables of high quality.
With the extrusion line of this type, a reduction in the RPM of the screw or an increase in the line speed while the extrusion temperature is kept constant results in a reduction in the outside diameter of the cable. When the extrusion temperature is increased, the expansion ratio of the insulation is increased and the outside diameter becomes larger. With the line speed going higher, the period of time required for the insulation to be cooled and solidified after extrusion is shortened, and hence the foaming process is finished early, resulting in a lower expansion ratio. Accordingly, these variable factors are related closely to each other. It is necessary to effect stable control of the outside diameter and electrostatic capacity (foaming ratio) while taking into account the correlation between these factors.
In multivariable control, a multiplicity of measured variables, including, in the illustrated arrangement, the outside diameter D of the cable, the electrostatic capacity C of the insulation, which are controlled variables, temperatures C3, C4 indicated on the cylinder-temperature sensors TM3, TM4, a temperature C6 indicated on the crosshead-temperature sensor TM6, a temperatue C5 indicated on the resin-temperature sensor TM5, and a pressure P indicated on the resin-pressure gage PM, are varied when any one of a multiplicity of manipulated variables including, in the illustrated arrangement, the RPM of the screw 20, the voltage applied to the preheater 16, the voltages applied to the cylinder heater units 23-3, 23-4, the voltages applied to the crosshead heater units 24-1, 24-2, the distance between the movable water tank 28 and the crosshead 21. With this type of multivariable control, it has conventionally been quite difficult by means of the measured variables to control the manipulated variables, simultaneously and automatically until the controlled variables reach desired values.
The control system in which the control variables and the manipulation variables are correlated in pairs cannot achieve quick response to and compensation for disturbances, with the consequences that the variables to be controlled are less stable and responsive. Since this control system does not control the electrostatic capacity C and the outside diameter D in response to the line speed, all of the products fabricated up to the point in which the line speed is rendered normal are useless. Furthermore, since the electrostatic capacity C and the outside diameter D are not constant until the resin temperature and the other temperatures are increased even during the normal operation, any cable manufactured before the required temperatures are reached is also useless. Therefore, the yield with the prior extrusion line has been poor.